Photoelectric smoke detectors are commonly used to detect the presence of smoke particles in the air by sensing the scattering of light energy by smoke particles in a sample chamber of the detector. The sample chamber is commonly made with walls which have a dull black finish to minimize light reflection. A source of light is positioned to shine a beam of light into the chamber. Should there be smoke present in the chamber, the beam of light energy is scattered by the smoke particles and a portion of such scattered light is sensed by photoelectric sensor, such as, a photodiode or the like, to cause an alarm upon sensing an amount of scattered light energy which results from a predetermined smoke concentration.
One of the concerns in using a photoelectric smoke detector is that the photoelectric sensor and the source of light, such as a light emitting diode and associated electronic circuitry, are prone to failure or change in characteristics over extended periods. It is, therefore, important to provide on the detector a type of test unit that, when actuated by the consumer, indicates the device is still functioning as intended.
Several approaches have been taken in the past to provide test units for photoelectric smoke detectors, such as that disclosed in U.S. Pat. No. 3,868,184. This patent discloses a particle simulating device for testing the operativeness of the photoelectric sensing cell and its associated alarm circuit. The particle simulating device comprises a thin wire mounted on a bell crank which is manually operated from outside of the detector to swing the wire through the detection zone at the intersection of lamp and photocell axis, so as to scatter light from lamp to the cell in the same way an amount of particles in the zone would accomplish. This arrangement lacks precision in simulating a predetermined smoke concentration, since it is very difficult to mount in the detector a sufficiently thin wire to simulate a light scattering which would be caused by a concentration of smoke in the range of 1.5 to 3% obscuration.
In an attempt to improve the precision of the mechanical device for simulating predetermined concentrations of smoke, a wire may be positioned to swing into the sensitive area of the chamber, which is not at the intersection of the lamp and photocell axis. This permits the use of a larger diameter wire to scatter light which has strayed from the light beam. The difficulty with this arrangement is that a very exact location is needed in positioning the wire in the sensitive zone to provide a consistent scattering of an amount of light. This entails the use of a complex mechanical linkage which will always ensure the exact positioning for the wire in the sensitive zone and must be such that the swinging movement of the wire is always the same and is not directly dependent upon the extent to which a test button is pressed by the consumer.
Another approach for testing photoelectric smoke detectors is disclosed in U.S. Pat. No. 4,099,178. In that instance, the test means includes a light source and light responsive device and has a separate test light path other than through the smoke chamber with light modifying means in the test path to simulate a preselected smoke density. The test means includes a normally closed gate for the test light path, the gate being manually opened to transmit light along the test path to the light responsive device to cause actuation of the alarm in the absence of smoke to indicate detector operability. This arrangement entails the incorporation into the device of a separate test light path other than the normal path and the use of a mechanical linkage or the like to provide externally of the device means for opening and closing of the gate which permits the light from the light source to travel along the test light path. The selection of the light modifying means in the test path is critical, because with variations in its characteristics, there is a result in variation in the sensitivity at which the detector is tested. Thus, to ensure a testing of the device in a desired sensitivity range, it is important to have rigid standards which must be met by the light modifying means, such as opaque tape, to maintain a test in desired sensitivity range.
With the above form of testing devices for smoke detectors, there is little, if any, flexibility in adjusting for changes in the characteristics of the detector components. This may arise, for example, in changing sources of supply for the photoelectric sensor, the light source or other electronic components. With mechanical devices for providing light scattering adjustment of the amount of light to be scattered is very difficult, if not impossible, with most units. Therefore, once the particular size and location of the mechanical test unit has been determined, it is important to continue with the same source of supply of the components to ensure that the manufactured units are tested at the same sensitivity.